Preheat the oven to 375°F. Line a square pan with parchment paper and set aside.
In a small bowl, mix together the milk and the vinegar. Set aside to “sour”.
In a mixing bowl, cream together the butter and sugar until light and fluffy. Add the egg, scraping the sides of the bowl.
In a separate bowl mix the flour, cocoa, baking soda, salt and cinnamon until fully combined.
Add the water to the “soured” milk.
Slowly alternate between mixing in the the dry ingredients and the milk ingredients to the butter mixture until fully incorporated.
Pour the batter into the prepared pan and bake in the preheated oven for 25-35 minutes or until a toothpick inserted in the middle comes out clean.
Leave to cool completely.
Beat the frosting ingredients together and frost cooled cake.